1. Field of the Invention
The present invention relates to a medium voltage switchgear having a circuit breaker, the circuit breaker having an operating mechanism with an opening solenoid and, more specifically, to an opening solenoid having additional mass coupled to the solenoid clapper, the additional mass structured to cause an inertial delay to the operation of the solenoid clapper.
2. Background Information
A medium voltage switchgear, typically, comprises a switching mechanism housed in an enclosure. The switching mechanism, typically a circuit breaker, includes a plurality of separable contacts coupled to an operating mechanism having a common spring-operated closing and tripping device. The operating mechanism includes one or more opening springs which separate the contact and a pair of closing springs which close the contacts as well as charge the opening spring. The separable contacts are closed by releasing the energy stored in the closing springs through activation of a closing trigger mechanism. This can be done manually or remotely through a solenoid. The closing springs are charged manually by a lever arm through a ratchet coupling, or, more preferably, by a motor. An electronic trip circuit monitors the load currents and actuates an opening trigger mechanism through an opening solenoid if the current exceeds certain current-time characteristics.
The opening solenoid includes an elongated clapper structured to move between a first position and a second position. The elongated clapper extends between the trip device and a trip lever in the operating mechanism. The clapper is, typically, made from steel. The trip lever is fixed to a D shaft that engages the operating mechanism trip latch. When the clapper is in the first position, the operating mechanism trip latch may engage the D shaft. When the trip latch is held by the D shaft, the separable contacts may be closed. Once the separable contacts are closed current may pass through the circuit breaker. If an external control device applies the appropriate voltage and current to the solenoid coil, or if the clapper is manually activated, the clapper moves to the second position. As the clapper moves into the second position, the clapper causes the trip lever to rotate which, in turn, causes the D shaft to rotate. As the D shaft rotates, the trip latch disengages from the D shaft and allows the operating mechanism to separate the contacts.
The opening solenoid has a mass limit. That is, the opening solenoid is structured to move a mass, the clapper, and that mass has a maximum limit. The higher the mass limit, the greater the mass the opening solenoid is structured to move. Generally, an opening clapper has a mass that is between 5% and 20% of the mass limit of an opening solenoid. For example, a typical opening solenoid has a mass limit of about 1.89 kg and a typical opening clapper has a mass of about 0.2 kg. In this configuration, the response time, that is the time to move the clapper between the first and second position, is about 10 ms to 35 ms, and more typically 25 ms. When the mass of the opening clapper is reduced, the response time, i.e., the time required for the clapper to move between first and second positions, of the opening solenoid is decreased. That is, with a lighter opening clapper, the opening solenoid clapper moves between the first and second positions more rapidly.
It is generally assumed that the response time of the opening solenoid should be as short as possible. That is, when the current exceeds certain current-time characteristics it is desirable to have the operating mechanism separate the contacts as quickly as possible to avoid, or minimize, damage to the circuit breaker and/or load side electrical components. To ensure that the operating mechanism responds rapidly, the opening solenoid must respond rapidly as well. However, it has also been determined that a typical over current, or “fault current,” situation includes a decaying direct current as well as an alternating current. That is, a current, either direct or alternating, has a wave form that may be expressed, generally, as a sine wave. A decaying direct current occurs just after the direct current wave form is at a peak. It has further been determined that, if the contacts are separated at, or near, the maximum wave peak, i.e., both the direct and alternating currents are at or near their peaks, the contacts may be damaged. Given that the fault current typically occurs when the direct current is at, or just past, a peak, separation of the contacts at the maximum wave peak could be avoided if the separation of the contacts was delayed until the direct current was off peak.
There is, therefore, a need for an operating mechanism structured to delay the separation of the contacts until the direct current was off peak.
There is a further need for an opening solenoid that is structured to delay the movement of the clapper thereby delaying the release of the operating mechanism trip latch.
There is a further need for an opening solenoid that is structured to delay the movement of the clapper that may be incorporated into existing circuit breakers.